[time-nuts] Got 60HZ?

[francesco messineo]

Hello,

On Thu, Dec 9, 2010 at 5:13 AM, Michael Poulos <poulosmd at gmail.com> wrote:
> Recently I bought a Efratom Ru frequency standard from eBay and a frequency
> divider chip that makes 1MHZ,100KHZ,25KHZ,10KHZ,100HZ and a 1HZ output.
> Today I thought of a way to make a nice 60HZ so you can use a mains-powered
> clock for the display (using amplifier and transformer wired "backwards").
> But, now you'll need 60HZ. A European has it easy with 50HZ as you use a
> BASIC Stamp or Arduino to divide the 100HZ output. But for 60HZ I came up
> with a solution:
>
> You set up the Arduino to take the 10KHZ from the divider chip and program
> it to count off 83 pulses to flip an output. But wait! Unless you add a
> "leap count" every 3 flips of the output, it'll run fast. Assume at the
> start the Arduino output starts high then turns low:
>
> (83+83+84+83+83+84)*20 = 10,000 pulses = one second
> H__L__H__L__H__L
>
> Every output cycle and a half the voltage swing is a little over 1 percent
> longer because of the leap count. This means that the distortion adds a
> slight inaccuracy, not enough to upset New Year's revelers. But if you want
> a better 60HZ, try using the 100KHZ:
>
> (833+833+834+833+833+834)*20 = one second
>
> You see where this is going with leap counts. The ultimate of course is one
> really good Arduino and (after a hex inverter to amplify it) take the
> straight 10MHZ and apply this leap count technique:
>
> (83333+83333+83334+83333+83333+83334)*20 = one really accurately made 60HZ =
> one nice second, just the thing for a Nixie clock. :)
>
> Now, what is a good hex inverter to take the 10 million HZ of my rubidiom
> movement to feed a frequency divider chip (and later Arduino)? It needs to
> take the .5 of a volt sinewaIe and squarewave it and in a normal 14 pin DIP
> (breadboardable) package.


if you are not afraid of a little microcontroller programming, why not
use a software DDS approach like this:


http://www.myplace.nu/avr/minidds/index.htm

it can output a nice sine wave at 60 Hz (or whatever) from say a 10
MHz clock really easily and the frequency is also easily tunable in
software.

I built several similar low frequency (audio range) software DDS using
AVR and other microcontrollers.
I usually add a one or two stage active low pass filter after the R/2R
network. I also used to build the R/2R network out of selected 1% 10K
resistors, final resistor match is usually good to 0.1% tolerance, but
in some boards I just put 5% parts.
Software is basicly an adder and you use the highest byte as a pointer
to the ram or rom waveform samples, once you understand how it works,
it's really easy to adapt to your needs. Usually the waveform you
obtain has a DC offset, but that's easily solved too.

Hope it helps.

Frank   IZ8DWF